The present invention relates generally to electronic circuits that include power devices, and, in particular, to mounting a power device to a heat spreader.
Engine mounted electronic control units for vehicular engines are subject to a high level of heat and vibration. Performance of the electronic components and a circuit substrate included in engine mounted electronic control units is often limited or impaired due to these conditions. In addition, electronic power devices, such as power transistors, dissipate energy in the form of heat when operating in an electronic control unit. The high ambient temperatures of an engine mounted electronic control unit in combination with the heat dissipated by an electronic power device imposes a thermal constraint on the operation of the device, since excessive operating temperatures can reduce device longevity and can damage the device. As a result, in order to reduce a device operating temperature and to enhance the device""s thermal performance, traditional cooling methods such as the use of a heat sink, or cooling plate, have been implemented.
The thermally conductive heat sink conducts heat generated by the power device away from the component, thereby helping regulate the operating temperature of the component. Often the heat sink is manufactured from a thermally and electrically conductive metallic material that also provides mechanical support for the electronic device and circuit substrate. As circuit substrates typically are poor conductors of heat, the power device is typically secured directly to the heat sink.
One method employed to secure a device to a heat sink is a mechanical clip that is screwed, at one end, into the heat sink. A second end of the clip is disposed on top of the power device and applies pressure to the power device in order to mechanically secure the device to the heat sink. However, assembly of such a clip is a mechanically intensive process and the securing of a power device to a heat sink merely by virtue of pressure provides a sub-optimal thermal path to the heat sink.
A more thermally efficient method for securing a power device to a heat sink is to indirectly solder bond the component to the heat sink. Solder will not bond directly to thermally conductive metallic materials, such as aluminum, that are commonly used to manufacture heat sinks. Therefore methods have been developed to indirectly bond a power device to the heat sink. One such technique employs a non-conductive organic laminate material as an interface layer, or pad, between the component and the heat sink as is described in U.S. Pat. No. 6,165,612, hereby incorporated by reference herein in its entirety. However, most organic laminate materials have very limited thermal conductivity. For example, the Bergquist T-clad material has a thermal conductivity of 3 watts/meterxc2x7xc2x0 C. Adhesives are then used to secure the component to the laminate and the laminate to the heat sink. This limits the use of such materials in high power device applications where several hundreds of watts of power may need to be dissipated in a short period of time.
Another technique for bonding a power device to a heat sink employs an inorganic oxide layer that is used to bond a solderable metal, typically copper, to a thermally conductive interface layer or pad, such as alumina or beryllium oxide. The inorganic oxide layer is grown on the interface layer and facilitates the affixing of a copper layer on top of the interface layer. The power device can then be soldered to the copper layer, and thereby to the interface layer. However, a non-electrically conductive oxide layer is normally obtained through high temperature processes such as direct bond copper (DBC) or active metal brazing (AMB) that may involve temperatures well in excess of 1000xc2x0 C.
In addition, the cost of processing an inorganic structure such as DBC or AMB is high, typically about $1 per square inch. Furthermore, in order to then attach the thermally conductive interface layer to the heat sink, the pad must be mechanically attached to the heat sink or another interface material, such an organic adhesive, must be used. This adds further cost to the process of securing the component to the heat sink and increases the overall thermal resistance between the component and the heat sink. Also, beryllium oxide is a high cost hazardous substance that imposes safety constraints on a design and manufacturing process and further creates disposal problems.
Therefore, a need exists for a method and apparatus for securing a power device to a heat sink, which method and apparatus are of lower cost than prior art methods, may be employed in high volume production, results in a low resistance thermal path between the component and the heat sink, and does not involve a hazardous substance, thereby all o wing for the dissipation by the component of large amounts of power in short intervals of time.